Implantable cardiac stimulator with electrode-tissue interface characterization

ABSTRACT

A cardiac stimulator capable of measuring pacing impedance includes a tank capacitor for delivering charge to the heart via device leads, a shunt resistor, and high-impedance buffers for measuring pacing current through the shunt resistor. Soon after the leading edge of the stimulation pulse, the voltage across the shunt resistor, as sampled by a high-impedance buffer, indicates lead and cardiac tissue resistance. Just prior to opening the pacing switch to terminate the stimulation pulse, the voltage across the shunt resistor is sampled by a high-impedance buffer and held once again to allow the capacitance of the lead/heart tissue to be calculated. In alternative embodiments, a high-impedance buffer measures the voltage between the tank capacitor and ground immediately following the stimulation pulse to allow estimation of the lead/heart tissue capacitance. In one alternative embodiment, a look-up table is created in main memory and searched to find the closest lead/heart tissue capacitance estimate to any arbitrary degree of accuracy. In another alternative embodiment, the lead/heart tissue capacitance is estimated by successive approximation to any arbitrary degree of accuracy. When the lead/heart tissue capacitance and lead resistance have been determined, a plurality of parameters of importance for analyzing and optimizing a cardiac stimulation system may be calculated, such as the instantaneous current, the average current, the charge, and the energy delivered to the cardiac tissue.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to implantable cardiac pacingsystems and particularly to an improved technique for electrode-tissueinterface characterization. More particularly, the present inventionrelates to an apparatus and method for measuring the resistive andcapacitive components of the impedance of pacemaker or defibrillatorleads.

Background of the Invention

In the normal human heart, illustrated in FIG. 1, the sinus (orsinoatrial (SA)) node generally located near the junction of thesuperior vena cava and the right atrium constitutes the primary naturalpacemaker by which rhythmic electrical excitation is developed. Thecardiac impulse arising from the sinus node is transmitted to the twoatrial chambers (or atria) at the right and left sides of the heart. Inresponse to excitation from the SA node, the atria contract, pumpingblood from those chambers into the respective ventricular chambers (orventricles). The impulse is transmitted to the ventricles through theatrioventricular (AV) node, and via a conduction system comprising thebundle of His, or common bundle, the right and left bundle branches, andthe Purkinje fibers. The transmitted impulse causes the ventricles tocontract, the right ventricle pumping unoxygenated blood through thepulmonary artery to the lungs, and the left ventricle pumping oxygenated(arterial) blood through the aorta and the lesser arteries to the body.The right atrium receives the unoxygenated (venous) blood. The bloodoxygenated by the lungs is carried via the pulmonary veins to the leftatrium.

This action is repeated in a rhythmic cardiac cycle in which the atrialand ventricular chambers alternately contract and pump, then relax andfill. Four one-way valves, between the atrial and ventricular chambersin the right and left sides of the heart (the tricuspid valve and themitral valve, respectively), and at the exits of the right and leftventricles (the pulmonic and aortic valves, respectively, not shown)prevent backflow of the blood as it moves through the heart and thecirculatory system.

The sinus node is spontaneously rhythmic, and the cardiac rhythm itgenerates is termed normal sinus rhythm ("NSR") or simply sinus rhythm.This capacity to produce spontaneous cardiac impulses is calledrhythmicity, or automaticity. Some other cardiac tissues possessrhythmicity and hence constitute secondary natural pacemakers, but thesinus node is the primary natural pacemaker because it spontaneouslygenerates electrical pulses at a faster rate. The secondary pacemakerstend to be inhibited by the more rapid rate at which impulses aregenerated by the sinus node.

Disruption of the natural pacemaking and propagation system as a resultof aging or disease is commonly treated by artificial cardiac pacing, bywhich rhythmic electrical discharges are applied to the heart at adesired rate from an artificial pacemaker. An artificial pacemaker (or"pacer") is a medical device which delivers electrical pulses to anelectrode that is implanted adjacent to or in the patient's heart inorder to stimulate the heart so that it will contract and beat at adesired rate. If the body's natural pacemaker performs correctly, bloodis oxygenated in the lungs and efficiently pumped by the heart to thebody's oxygen-demanding tissues. However, when the body's naturalpacemaker malfunctions, an implantable pacemaker often is required toproperly stimulate the heart. An in-depth explanation of certain cardiacphysiology and pacemaker theory of operation is provided in U.S. Pat.No. 4,830,006.

Pacers today are typically designed to operate using one of threedifferent response methodologies, namely, asynchronous (fixed rate),inhibited (stimulus generated in the absence of a specified cardiacactivity), or triggered (stimulus delivered in response to a specifiedhemodynamic parameter). Broadly speaking, the inhibited and triggeredpacemakers may be grouped as "demand" type pacemakers, in which a pacingpulse is only generated when demanded by the heart. To determine whatpacing rate is required by the pacemaker, demand pacemakers may sensevarious conditions such as heart rate, physical exertion, temperature,and the like.

Moreover, pacemaker implementations range from the simple fixed rate,single chamber device that provides pacing with no sensing function, tohighly complex models that provide fully automatic dual chamber pacingand sensing functions. The latter type of pacemaker is the latest in aprogression toward physiologic pacing, that is, the mode of artificialpacing that most closely simulates natural pacing.

Referring now to FIG. 2, a conventional implantable medical device 200is shown implanted and coupled to a patient's heart 250 by leads 205 and210. The implantable medical device 200 may include a pacemaker ordefibrillator or any medical device that performs pacing ordefibrillating functions. The implanted medical device 200 (or simply"pacer") also includes housing or "can" 215 which houses a battery andpacing or defibrillating circuitry (not shown). In the dual chamberpacing arrangement shown, leads 205 and 210 are positioned in the rightventricle and right atrium, respectively. Each lead 205 and 210 includesat least one stimulating electrode for delivery of electrical impulsesto excitable myocardial tissue in the appropriate chamber(s) in theright side of the patient's heart. As shown in FIG. 2, each lead 205 and210 includes two electrodes. More specifically, lead 210 includes ringelectrode 230 and tip electrode 235, and lead 205 includes ringelectrode 220 and tip electrode 225. Two, three, and four terminaldevices all have been suggested as possible electrode configurations.

A lead configuration with two electrodes is known as a "bipolar lead."Such a configuration typically consists of a pair of wires arrangedcoaxially and individually insulated. Each of the wires may consist ofmultiple wire strands wrapped together for redundancy. A circuitconsisting of the pacemaker 200 and the heart muscle can be formed byconnecting the lead electrodes to different portions of the heartmuscle. In a bipolar configuration, electric current impulses generallyflow from the ring electrode through the heart muscle to the tipelectrode, although current may travel from the tip electrode to thering electrode in alternative configurations. A lead with one electrodeis known as a "unipolar lead." In a unipolar configuration, thepacemaker can 215 functions as an electrode. Current flows from theunipolar lead through the heart tissue, returning to the pacer via thecan 215.

In general, a pacing pulse current is formed by the flow of chargecarriers in the circuit formed by the lead and tissue. Because theelectrode is typically composed of a solid conductive material, whilethe myocardial tissue consists of liquid electrolyte, the electrodeforms an electrode/electrolyte interface through which the chargecarriers pass. Accordingly, electron conductivity accounts for chargetransfer in the lead circuit and in the solid phase of the electrodeinterface, while ion conductivity is the primary mechanism responsiblefor charge flow through the electrolyte interface and tissues.

At the interface layer, pacing pulse charge flows from the solid phaseof the electrode interface to the electrolyte phase until theelectrochemical potential of the electrode interface balances theelectrochemical potential of the electrolyte interface. During such aprocess, an electric charge layer, known as the Helmholtz layer, formsaround the surface of the electrode. While the exact nature of theHelmholtz layer is very complex, it can be generally modeled as anelectric circuit using voltage sources, diodes, and/or devices thatcontribute impedance (which is the ability to impede electric current)to the lead-tissue circuit. Electrical impedance may be generallycharacterized by the combination of a resistive component, such as aresistor, with a reactive component, such as a capacitor or inductor.One Helmholtz layer model includes a polarization potential (known asthe "Helmholtz voltage") in series with the parallel combination of aresistor (known as the "Warburg resistor") and a capacitor (known as the"Helmholtz capacitor"). A second Helmholtz layer model has beensuggested which consists of an impedance circuit shunted by two zenerdiodes. The second configuration accounts for the electrical behavior ofheart tissue when the interface voltage exceeds several hundredmillivolts. A simple yet accurate model of the Helmholtz layer consistsof the Warburg resistance in series with a voltage-dependent Helmholtzcapacitance, eliminating the need to model the polarization potential.

FIG. 3A illustrates a model of a conventional cardiac stimulator circuitconsisting of a pacer 200, heart tissue 250, and bipolar pacer lead 205terminated by tip electrode 225 and ring electrode 220. Ring electrode220 and tip electrode 225 couple the pacer 200 to different portions ofthe heart tissue 250. Alternatively, a model as in FIG. 38 using aunipolar lead 305 would include a single electrode 320 coupled to theheart tissue 250 with the pacer can 215 coupled to the chest tissue,labeled as ground. In the unipolar configuration of FIG. 3B, the pacer200 sends electric current from the pacer can 215 to a single electrode320 through the chest and heart tissue 250. Accordingly, the impedanceintroduced by the combination of chest tissue (FIG. 3B only), bipolarlead 205 or unipolar lead 305, and heart tissue 250 may be collectivelymodeled by resistor R3 (the Warburg resistor) in series with capacitorC3 (the Helmholtz capacitor).

Such models as shown in FIG. 3A and 3 B are important for delivering"pacing impedance" estimates, which help to indicate the condition ofthe pacer leads as well as to estimate electric charge, current, andenergy delivered to the heart tissue. Particularly, deviations thatoccur over time in the pacing impedance serve to indicate the conditionsrelated to the pacing or defibrillation lead system. Such conditionsinclude electrode micro-dislocation, lead impedance changes, evaluationof electrode suitability for detecting evoked potentials, and methodsfor detecting changes in the excitable tissue as a function ofcatecholamine concentration, metabolic changes, and ischemia. Inaddition, the charge, current, energy, and impedance measurements allowphysicians to estimate the longevity of the implanted device.Accordingly, pacing impedance estimates aid physicians in maintainingand optimizing pacemaker operation throughout the life of the device.

Although a purely resistive lead impedance estimate may provide a meansfor a rough estimate of pacer and battery condition, such an estimatemay deviate significantly from the true impedance in some situations,since the physical and electrochemical properties that lead to theHelmholtz layer change with variations in the electric field intensitywhich develops at the electrode-electrolyte interface. For example,corrosion, electrocatalysis of glucose and amino acids, and hydrogen ionpotentiodynamics drastically alter the modeled capacitance, resistance,and polarization of the interface, as do electrode current density andelectric field strength. Further, the Helmholtz capacitance variesaccording to a parameter known as the "microsurface area" of theelectrode. The microsurface area of the electrode is the total surfacearea of the electrode material, including microscopic details such asporosity and other microscopic details. Typically, the Helmholtzcapacitance equals about 100 microfarads (pF) per square centimeter ofmicrosurface area. In addition, the resistance, capacitance, andpolarization voltage of the Helmholtz layer can vary according to theduration and amplitude of the pacing pulse, although these propertiesare approximately constant for pulse widths of less than 0.5milliseconds (ms) and pulse amplitudes of less than 0.5 volts (V).

Methods for measuring the resistive component of pacing impedance havebeen available for some time as part of the information that implantablepacemakers and defibrillators can collect and telemeter. However, suchestimates have neglected the reactive impedance component, as modeled bythe Helmholtz capacitance, resulting in an incomplete characterizationof the pacing impedance. Such omissions produce undesirable impedanceestimation errors which may propagate into subsequent calculations ofcharge, current, and energy delivered to the heart tissue as well asother conditions closely related to the pacing impedance.Impedance-based methods for monitoring the leads and electrodes ofimplantable cardiac stimulators have been described in a number ofpatents, including U.S. Pat. No. 4,899,750, U.S. Pat. No. 5,201,865, andU.S. Pat. No. 5,534,018 which disclose devices for estimating theresistive lead impedance component.

While measurement of the Helmholtz capacitance has been suggested usingalternating current (AC) circuits, such circuits are not practical foruse with cardiac stimulation devices, which typically use direct current(DC) pulses for cardiac stimulation. Accordingly, devices using ACmethods must operate exclusively of normal pacemaker/defibrillatoroperation. Therefore, no practical device or method for estimating boththe resistive and reactive components of pacer lead impedance has beendevised within a cardiac stimulator, and present-day cardiac stimulatorsmust tolerate the inaccuracies introduced by purely resistive impedanceestimates, as described above.

For the foregoing reasons, a practical apparatus for measuring both theresistive and capacitive components of the lead impedance, including theHelmholtz layer, would greatly improve the implementation of implantedstimulation devices. Such an apparatus, if devised, should be adapted tomeasure lead impedance during normal operation of the implanted devicewithout affecting the functionality of the pacing or defibrillatingcircuit. The resulting device would significantly improve the accuracyof cardiac impedance estimates, resulting in superior optimization andmaintenance of implanted devices. Unfortunately, to date, no such deviceis known that provides these features.

SUMMARY OF THE INVENTION

Accordingly, there is provided herein a cardiac stimulator including apulse generator for delivering current to the heart tissue, an impedancemeasurement circuit coupled to the pulse generator, and a processor forperforming control and calculation functions. Upon receiving controlsignals from the processor, the pulse generator transmits electriccurrent (known as a pacing pulse) from a charged capacitor into theheart tissue. At the same time, the processor asserts control pulses tothe impedance circuit, causing the impedance circuit to sample voltagesfrom the pulse generator. The impedance circuit records the voltagemeasurements through sample-and-hold units, transmitting the voltages assignals to the processor. Using these voltage measurements, theprocessor calculates the impedance of the lead/tissue circuit.

The pulse generator includes a tank capacitor for delivering charge tothe heart via device leads and a pacing voltage source for charging thetank capacitor through an electronically-controlled charge switch. Justprior to the that the pacing pulse is to be delivered to the hearttissue, the charge switch is opened. A pacing switch is then closed toallow charge from the tank capacitor to flow through a DC-blockingcapacitor into the lead and subsequently the heart. Opposing the flow ofthis current are the resistance of the pacing switch, the resistivecomponents of the lead and load impedance (i.e., the lead resistance andionic resistance), the Helmholtz capacitance, and acurrent-measurement-shunt resistor.

Soon after the leading edge of the pacing pulse, or at time t=(0⁺), thevoltage across the current-measurement-shunt resistor is sampled througha high-impedance buffer and held. Since the DC-blocking and Helmholtzcapacitances have not charged appreciably at t=(0⁺), they behave asshort-circuits. The pacing circuit is therefore purely resistive, andthe lead and ionic resistance may be calculated by the method of circuitanalysis.

Just prior to opening the pacing switch to terminate the pacing pulse,or at time t=(T_(PW) ⁻), the voltage across thecurrent-measurement-shunt resistor is sampled by a high-impedance bufferand held once again to allow the Helmholtz capacitance to be calculated.After the pacing pulse is delivered and before the tank capacitor isrecharged, the end voltage of the tank capacitor is sampled through ahigh-impedance buffer and held. Concurrently with the sampling of thetank capacitor end voltage, the DC-blocking capacitor discharges intothe human body by an active discharge switch and a passive-dischargeresistor. In a preferred embodiment, the resistive and capacitivecomponents of the lead impedance may be calculated explicitly using theshunt resistor voltage samples from the high-impedance buffers.

In other embodiments, the apparatus estimates the Helmholtz capacitancewithout knowledge of the voltage across the current-measurement-shuntresistor just prior to the end of the pulse. The voltage across the tankcapacitor after the pulse ends, i.e. at t=(T_(PW) ⁺), may be expressedusing a formula based on pacing voltage, tank capacitance, DC-blockingcapacitance, Helmholtz capacitance, current-measurement-shuntresistance, pacing switch resistance, lead/tissue resistance, and pulsewidth, all of which are known values except the Helmholtz capacitanceand lead/tissue resistance. The tank voltage formula consists of anexponential term multiplied by a constant term and added to an additiveterm. All three terms include the Helmholtz capacitance as a variable.If the tank capacitor voltage is measured following the pulse and thelead/tissue resistance is calculated using circuit analysis as above,then the formula reduces to an equation involving only one unknownvariable, the Helmholtz capacitance.

In an alternative embodiment, a look-up table is created in main memoryby using the calculated Warburg resistance combined with known values ofthe pacing voltage, tank capacitance, DC-blocking capacitance,current-measurement-shunt resistance, pacing switch resistance, andpulse width in the formula along with a series of empirical estimatesfor the value of the Helmholtz capacitance. The formula produces adistinct tank capacitor voltage calculation for each Helmholtzcapacitance estimate. The Helmholtz capacitance estimates along with thecalculated tank capacitor voltages are stored into main memory as alook-up table, and the actual, measured tank capacitor voltage iscompared with the set of calculated tank capacitor voltages. Searchingthrough the look-up table, the apparatus chooses the Helmholtzcapacitance estimate as the empirical estimate which produced acalculated tank capacitor voltage that most closely resembles themeasured tank capacitor voltage.

In another embodiment, a single empirical estimate for the Helmholtzcapacitance is substituted into the one part of the formula, either intothe exponential term or into the additive and constant terms. Theremaining term(s) may be reduced algebraically to solve for the unknownHelmholtz capacitance value. If the resulting calculation of theHelmholtz capacitance value does not agree with the originallysubstituted empirical estimate, then an updated empirical estimate issubstituted into the first term(s), and a new Helmholtz capacitance iscalculated using the remaining term(s). If the resulting calculation ofthe Helmholtz capacitance value lies within an acceptable range of theoriginally substituted empirical estimate, then the measured Helmholtzcapacity is determined as the final empirical estimate. Such anapproximation is simple to compute using conventional circuitry and canconform to any arbitrary level of accuracy by iterating through theequation with progressively better estimates for the Helmholtzcapacitance.

When the Helmholtz capacitance and Warburg resistance have beendetermined, a plurality of parameters of importance for analyzing andoptimizing a pacing system may be calculated, including the currentdelivered to the cardiac tissue at any instantaneous point in time, theaverage current delivered to the cardiac tissue over the duration of thepulse, the total charge and the total energy delivered to the cardiactissue and to the leads, and the Helmholtz potential after pacingpolarization.

Thus, the present invention comprises a combination of features andadvantages that enable it to substantially advance the art by providingan apparatus for gauging both the resistive and capacitive components ofthe Helmholtz layer. These and various other characteristics andadvantages of the present invention will be readily apparent to thoseskilled in the art upon reading the following detailed description ofthe preferred embodiments of the invention and by referring to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 illustrates the human heart;

FIG. 2 shows the typical connections between a conventionalpacer-defibrillator and the human heart;

FIG. 3A is a known model of the Helmholtz circuit for a bipolar leadconfiguration;

FIG. 3B is a known model of the Helmholtz circuit for a unipolar leadconfiguration;

FIG. 4 is an exemplary block diagram of a cardiac stimulator made inaccordance with the present invention;

FIG. 5 is a block diagram of the impedance circuit and pulse generatorcircuit of the cardiac stimulator shown in FIG. 4;

FIG. 6 is a timing diagram showing the control signals asserted by theprocessor of the cardiac stimulator shown in FIG. 4;

FIG. 7 is a graph of the voltage across the tank capacitor of FIG. 5versus the Helmholtz voltage created in the heart tissue during cardiacstimulation; and

FIG. 8 is a flowchart describing an alternative embodiment forestimating the Helmholtz voltage using the apparatus of FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An exemplary cardiac stimulator 400 made in accordance with the presentinvention is illustrated in the block diagram of FIG. 4. The cardiacstimulator 400 may be a pacemaker, a defibrillator, or any implantablecardiac stimulator. The cardiac stimulator 400 generally includes atrialand ventricular sense circuits 462 and 464, a processor 470, main memory475, an impedance circuit 466, and a pulse generator 468, all housed inan enclosure, or "can" 401. The exemplary embodiment of FIG. 4 showscardiac stimulator 400 with four leaded electrodes, namely atrial tipand ring electrodes 410 and 420, respectively, and ventricular ring andtip electrodes 440 and 450, respectively. Can 401 may function as anadditional electrode in accordance with known techniques. The invention,however, may be practiced using any number of electrodes implanted inany chamber of the heart and in any configuration.

Referring still to FIG. 4, electrodes 410 and 420 couple to the atrialsense circuit 462 via capacitors C1 and C2, respectively, which arepreferably 0.15 microfarad (μF) capacitors. Similarly, electrodes 440and 450 couple to the ventricular sense circuit 464 via capacitors C3and C4, respectively, which are also preferably 0.15 μF capacitors. Theatrial sense circuit 462 processes signals received from the atrialcharfiber of the heart via the atrial electrodes 410 and 420, while theventricular sense circuit 464 processes signals received from theventricular chamber via the ventricular electrodes 440 and 450. Theatrial and ventricular sense circuits 462 and 464 generally include alow power, highly sensitive amplifier, a band pass filter, and athreshold detector (not shown). The atrial 462 and ventricular 464circuits further include internal pulldown switches SW_(A) and SW_(V),respectively, the states of which are controlled by the processor 470.The amplifier amplifies the electrical signal from the associatedelectrodes, and the band pass filter attenuates signals whosefrequencies are outside the range of frequencies known to correspond tocardiac signals. The threshold detector compares the amplified andfiltered signal to a reference signal to determine when a cardiac event(also referred to as a "sense event") has occurred. If the magnitude ofthe amplified and filtered cardiac signal exceeds the reference signal,the processor 470 determines that a sense event has occurred. Theprocessor 470 may then pace the heart based either on detecting or notdetecting sense events via pulse generator 468 and electrodes 401, 410,420, 440, and 450. For example, the processor 470 may initiate aventricular pacing pulse if an atrial sense event has not been detectedwithin a predetermined period of time following a previous atrial senseevent.

Cardiac stimulator 400 further includes lead switches SW1 and SW2 aswell as can switch SW3 for configuring unipolar and bipolar sensingmodes and also unipolar and bipolar pacing modes, as described below.Switches SW1, SW2, and SW3 are preferably processor-controlled,single-pole single-throw (SPST) switches. When closed by the processor470, the atrial lead switch SW1 couples the atrial ring electrode 420 toground. Similarly, the ventricular lead switch SW2, when closed by theprocessor 470, couples the ventricular ring electrode 450 to ground. Canswitch SW3, when closed by the processor 470, couples the can 401 toground.

For atrial sensing using bipolar leads, atrial lead switch SW1, atrialinternal pulldown switch SW_(A), and can switch SW3 are all preferablyopen. In this configuration, the atrial sense circuit 462 receives adifferential sense signal from tip 410 and ring 420 electrodes,respectively. For atrial sensing using a unipolar lead configuration,atrial lead switch SW1 remains open, but atrial internal pulldown switchSW_(A) and atrial can switch SW3 are preferably closed.

Ventricular sensing operates in substantially the same manner. Forventricular sensing using bipolar leads, ventricular lead switch SW2,ventricular internal pulldown switch SW_(V), and can switch SW3 are allpreferably open. In this configuration, the ventricular sense circuit464 receives a differential sense signal from tip 440 and ring 450electrodes, respectively. For ventricular sensing using a unipolar leadconfiguration, ventricular lead switch SW2 remains open, but ventricularinternal pulldown switch SW_(V) and can switch SW3 are preferablyclosed.

The pulse generator 468 produces an appropriate electrical pulse tostimulate the desired chamber of the heart to beat. The processor 470initiates the pulse generator 468 to produce a pacing pulse, and thepulse generator responds by delivering the pacing pulse to the desiredchamber of the heart. The pulse generator 468 preferably includes a ratelimiter to prevent the processor 470 from erroneously pacing the heartat an excessively high rate. The pulse generator 468 preferably couplesto the atrial tip electrode 410 via an atrial pulse line 480 in serieswith a DC-blocking series capacitor C5 and further couples toventricular tip electrode 440 via a ventricular pulse line 485 in serieswith a DC-blocking series capacitor C6. Further, the pulse generator 468couples to ground to provide a circuit return path for pacing pulses.Hence, the pulse generator 468 may send a pacing pulse to the atrial orventricular chamber via atrial pulse line 480 or ventricular pulse line485, respectively.

In addition to selecting atrial or ventricular sensing, switches SW1,SW2, and SW3 configure the cardiac stimulator 400 for unipolar orbipolar pacing. For atrial bipolar pacing, atrial lead switch SW1 ispreferably closed (therefore coupled to ground), and can switch SW3 isopen. This bipolar pacing configuration allows a pacing pulse issued tothe atrial chamber via atrial pulse line 480 and atrial tip electrode410 to complete a circuit path to the pulse generator 468 through atrialring electrode 420, which couples to ground. Ventricular bipolar pacingoccurs in substantially the same manner, with ventricular lead switchSW2 closed (therefore coupled to ground) and can switch SW3 open. Apacing pulse issued to the ventricular chamber via ventricular pacingline 485 is then allowed to complete a circuit path to the pulsegenerator 68 through ventricular ring electrode 450, which couples toground.

For unipolar stimulation, can switch SW3 is closed, and atrial leadswitch SW1 (for stimulation of the atrial chamber) or ventricular leadswitch SW2 (for stimulation of the ventricular chamber) is opened. Inthis unipolar pacing configuration, a pacing pulse issued to the atrialchamber via atrial pacing line 480 and atrial tip electrode 410 isallowed to complete a circuit path to the pulse generator 468 via thecan 410, which is coupled to ground. Similarly, a pacing pulse issued tothe ventricular chamber via ventricular pacing line 485 and ventriculartip electrode 450 is allowed to complete a circuit path to the pulsegenerator 468 via the can 410, which is coupled to ground.

Main memory 475 couples to the processor 470 and is capable of storingprogram instructions and other data to be retrieved or updated by theprocessor 470. Accordingly, cardiac stimulator 400 may be programmedthrough instructions stored in main memory to operate in one of a numberof pacing modes. For example, the cardiac stimulator 400 may beprogrammed to sense electrical activity in the atrium, and then to pacethe ventricle following a predetermined time delay after the occurrenceof an atrial sense event if the ventricle has not contracted on its own.Additionally, the processor 470 may be programmed to store sense data,impedance data, or other information in main memory 475 to be retrievedat a later date either by the processor 470 or by a physician.

Cardiac stimulator 400 uses an impedance circuit 466 to determine theelectrical impedance of the lead and heart tissue circuit, as modeled byFIGS. 3A and 3 B. The impedance circuit 466 generally processes theelectrical signal from the pulse generator 468 and provides one or moreoutput status signals to the processor 470. The processor 470 uses thestatus signal from the impedance circuit 466 to compute the impedance ofthe lead/heart tissue, as described in more detail below.

FIG. 5 illustrates the electrical characteristics of the resistance oflead 505 combined with the impedance inherent in heart 250. ResistorR_(L) generally represents the combined resistance of the lead 505 andthe heart 250, while CL represents the Helmholtz capacitance describedpreviously. Note that R_(L) and C_(L) do not depict actual components inthe present invention but represent a model of the lead/heart tissueimpedance to be determined. Cardiac stimulator 400 calculateslead/tissue resistance R_(L) and Helmholtz capacitance C_(L) inaccordance with the methods described below. Referring still to FIG. 5,a preferred embodiment of a pulse generator 468 is shown coupled toheart 250 via lead 505. Pulse generator 468 comprises a voltage sourceV_(i), a charge switch SW1, a pacing switch SW2, tank capacitor C_(T),current-measurement-shunt resistor R_(T), a discharge switch SW3,discharge resistor R_(X) and DC-blocking capacitor C_(B).

Voltage source V; is any suitable voltage source for charging tankcapacitor C_(T).Voltage source V_(i) typically comprises a battery whichmay or may not be rechargeable and a programmable voltage multiplier.Voltage source V_(i) couples to charging switch SW1, which preferably isa singlepole/single-throw (SPST) switch controlled by a processor suchas processor 470 in FIG. 4, via a charge control signal 525. Tankcapacitor C_(T) and shunt resistor R_(T) couple in series betweencharging switch SW1 and ground, with C_(T) connected directly to SW1 andR_(T) connected directly to ground. One terminal of pacing switch SW2connects between charge switch SW1 and tank capacitor C_(T) while theother terminal of switch SW2 connects to a DC-blocking capacitor C_(B),discharge switch SW3, and discharge resistor R_(X). Pacing switch SW2 ispreferably an SPST switch with an internal switch resistance R_(SW).Processor 470 controls the state of pacing switch SW2 via a pace controlsignal 530. Switch SW3 likewise is a processor-controlled, SPST switch,coupling to the processor 470 via a discharge control signal 535.Discharge switch SW3 and discharge resistor R_(X) further couple inparallel and connect to ground. Discharge resistor R_(X) preferably hasa very high resistance compared with shunt resistor R_(T), switchresistance R_(SW), and lead/tissue resistance R_(L). A preferredembodiment includes a shunt resistor R_(T) of 22 Ω (ohms), a switchresistance R_(SW) of 10 Ω, a discharge resistor R_(x) of 100 k Ω, and atypical lead/tissue resistance of 500 Ω.

Lead 505 couples to DC-blocking capacitor C₈ and terminates to electrode520 at the heart 250. While lead 505 preferably comprises either abipolar or unipolar lead, it is illustrated in FIG. 5 as a unipolar leadfor simplicity. As one of ordinary skill in the art would recognize, thecircuits of FIGS. 3A and 3B are substantially the same, since the groundnode essentially serves as a lead substitute by providing a current pathfrom the cardiac stimulator 400 to the heart. Thus, the circuit of FIG.5 applies equally to both bipolar and unipolar lead configurations.

Impedance circuit 466 preferably comprises three sample-and-hold unitsU1, U2, and U3, as well as a pair of high-impedance buffers U4 and U5.Each buffer U4 and U5 may comprise any buffer circuit configured as avoltage follower with high-impedance inputs. The buffers U4 and U5 inthe present embodiment are shown as unity-gain operational amplifiers(or "op-amps"), with each buffer output coupled directly to theinverting input (-) of the same buffer. Alternatively, the buffers mayconsist of any device that amplifies an input signal. The invertinginputs of buffers U4 and U5 connect to resistors R1 and R2,respectively, which also couple to ground. The noninverting input (+) ofbuffer U4 couples to tank capacitor C_(T), charging switch SW1, andpacing switch SW2. The noninverting input of buffer US couples to thejunction between tank capacitor C_(T) and shunt resistor R_(T). Theoutput of buffer U4 drives the input of sample-and-hold unit U1. Theoutput of buffer U5 drives both sample-and-hold units U2 and U3.

The sample-and-hold units are controlled by the processor via signalssample1 540 (U1), sample2 545 (U2), and sample3 550 (U3). When a samplecontrol signal 540, 545, or 550 is asserted or pulsed, the correspondingsample-and-hold unit instantaneously samples the voltage appearing onits input terminal and holds that voltage on its output terminal evenafter the input signal is changed or removed. As described below, theoutput signals from sample-and-hold units U1, U2, and U3 representvoltages measured in the pulse generator 468. In a preferred embodiment,voltages are sampled at specific times in relation to the pacing pulse.For a pacing pulse with a duration of T_(PW) seconds, sample-and-holdunit U3 will sample the shunt resistor voltage just after the beginningof the pacing pulse, sample-and-hold unit U2 will sample the shuntresistor voltage just before the end of the pacing pulse, andsample-and-hold unit U1 will sample the tank capacitor voltage followingthe pacing pulse. A more detailed explanation of these voltages readingsis presented below, with respect to FIG. 6. The high-impedance nature ofbuffers U4 and U5 insures that the pulse generator 468 voltages aremeasured with negligible interference to the pulse generator 468.

Still referring to FIG. 5, voltage source V_(i) charges tank capacitorC_(T) to a voltage substantially equivalent to V_(i) when the chargingswitch SW1 is closed. When the charging switch SW1 and dischargingswitch SW3 are opened and pacing switch SW2 is subsequently closed, thetank capacitor C_(T) and shunt resistor R_(T) are effectively switchedinto a resistive-capacitive (or "RC") charging circuit including switchresistance R_(SW), discharge resistor R_(x), DC-blocking capacitorC_(B), lead/tissue resistance R_(L), and Helmholtz capacitance C_(L).Thus, the charge stored in C_(T) discharges into R_(T), R_(SW), R_(X),C_(B), R_(L), and C_(L).

FIG. 6 illustrates a detailed timing diagram of the control signalssample1, sample2, sample3, pace, discharge, and charge which areasserted by the processor 470 of FIG. 5 to control the pulse generator468 and impedance circuit 466. In the diagram of FIG. 6, the pacingpulse begins at t=0 and preferably extends for a duration of T_(PW)seconds. Prior to the beginning of the pacing pulse, the charge anddischarge signals are held low, or asserted, causing the charging switchSW1 and discharging switch SW3 to close. Also prior to the beginning ofthe pacing pulse, the pace signal is held high, or deasserted, causingthe pacing switch SW2 to open. Thus, the tank capacitor C_(T) charges toV_(i) volts. In a preferred embodiment, sampler, sample2, and sample3remain low prior to the beginning of the pacing pulse at time t=0,indicating that the previous samples are being held at the outputs ofsample-and-hold units U1, U2, and U3. The tank capacitor C_(T) becomessufficiently charged prior to time t=0, and the processor 470 deassertsthe charge and discharge signals at points 600 and 605, respectively.

The pacing pulse begins at time t=0 when the processor 470 asserts thepace signal (point 610) to a logic low state, allowing charge from thetank capacitor C_(T) to begin flowing into the lead/tissue circuit. Attime t=0+, which preferably is less than 10 μs after time t=0, theprocessor 470 pulses sample3 (point 615), causing sample-and-hold unitU3 to record the voltage V_(RT) (0⁺) across the shunt resistor R_(T).The tank capacitor C_(T) continues to discharge until the end of thepacing pulse at time t=T_(PW), which is marked by point 630. At timet=T_(PW) ⁻, however, which preferably occurs approximately 10 μs or lessbefore time t=T_(PW), the processor 470 pulses sample2 (point 620),causing sample-and-hold unit U2 to record the voltage V_(RT) (T_(PW))across the shunt resistor.

At time t=T_(PW), the processor 470 halts the pacing pulse bydeasserting the pace signal (point 630) to a logic high state.Subsequently, the electric charge accumulated in the DC-blockingcapacitor C_(B) and the Helmholtz layer (represented by C_(L)) begins todischarge to ground through the discharge resistor R_(X). In alternativeembodiments, the processor pulses sampler (point 635) at time T_(PW) ⁺,which preferably occurs approximately 10 μs or less after time t=T_(PW).Next, the processor 470 asserts the discharge and charge signals atpoints 640 and 645, respectively. The discharge signal allows anyelectric charge remaining in the DC-blocking capacitor C_(B) andHelmholtz layer (C_(L)) to quickly discharge, while the charge signalcauses voltage source V_(i) to charge tank capacitor C_(T) inpreparation for delivering the next pacing pulse.

Any capacitor behaves as a short-circuit for a short time after currentis applied to that capacitor. Thus, immediately after tank capacitorC_(T) and shunt resistor R_(T) are switched into the charging circuit,or at time t=0⁺, the current in the charging circuit equals the voltageheld by C_(T) divided by the resistance presented by the resistivecircuit of R_(X), R_(T), R_(SW), and R_(L). At the same time, processor470 asserts control signal sample3, causing sample-and-hold unit U3 tosample and hold the voltage drop V_(RT) (0⁺) across shunt resistorR_(T). Because the voltage drop across any resistor is proportional tothe current flowing through that resistor, the voltage V_(RT) (0⁺) canbe used to determine the current flowing through the charging circuit.It follows that the lead/tissue resistance R_(L) can be calculated usingequation (1) below: ##EQU1##

When a constant voltage is applied to an RC circuit, the amount ofcurrent flowing through that circuit changes over time in awell-documented manner. Thus, as the charge contained in tank capacitorC_(T) is released into the charging circuit from time t=0 to timet=T_(PW), the charging current changes over time. The rate at which thecurrent changes is determined by the resistances R_(T), R_(SW), andR_(L) and capacitances C_(T), C_(B), and C_(L).

Because the voltage drop across the shunt resistor at any point in timeV_(RT) (t) is directly proportional to the current through R_(T) andbecause the resistances R_(T), R_(SW), and R_(L) and capacitances C_(T),C_(B), and C_(L) uniquely determine the charging current at timet=T_(P), the Helmholtz capacitance C_(L) may be calculated usingequation (2) below. Because R_(X) has a very high impedance comparedwith the remaining components in the circuit, little current flowsthrough R_(X). Thus, the presence of R_(X) may be neglected for purposesof analyzing the Helmholtz capacitance C_(L). ##EQU2## where In() is thenatural logarithm function. Following the charging pulse,sample-and-hold units U3 and U2 hold voltages V_(RT) (0⁺) and V_(RT)(T_(PW) ⁻), respectively. Using these measured values of V_(RT) (0⁺) andV_(RT) (T_(PW) ⁻) along with known values of C_(T), R_(T), and R_(SW),the processor 470 calculates the lead/tissue resistance R_(L) and theHelmholtz capacitance C_(L) using equations (1) and (2), above. Thesecalculations provide an accurate characterization of the lead/tissueimpedance and assist physicians in monitoring lead integrity, devicelongevity, and current, charge, and energy delivered to the hearttissue.

The pulse generator 468 operates as described previously, and theprocessor 470 asserts sample3 at time t=0⁺ to measure the shunt resistorvoltage V_(RT) (0⁺) at the beginning of the pulse period. Shortly aftertime t=T_(PW), or at time t=T_(PW) ⁺, the processor 470 asserts thesampler control signal to cause and sample-and-hold unit U1 to recordthe voltage of tank capacitor C_(T) via buffer U4 immediately followingthe pulse period. The time t=T_(PW) ⁺ is preferably less than 10 μsafter time t=T_(PW). The tank capacitor voltage at time t=T_(PW) ³⁰ , orV_(CT) (T_(PW) ⁺), represents the voltage across tank capacitor C_(T)with respect to ground shortly after the pulse period. The measurementof V_(RT) (0⁺) allows the processor 470 to calculate the lead/tissueresistance R_(L) as before, using equation (1). In the alternativeembodiment, however, the processor 470 uses V_(CT) (T_(PW) ⁺) inequation (3), below, to estimate the Helmholtz capacitance C_(L) eitherby generating a lookup table or by successive approximation, as will beexplained below with respect to FIGS. 8A and 8B. Equation (3) governsthe tank capacitor voltage at time t=T_(PW) ⁺. ##EQU3## where e is thebase of the natural logarithm.

FIG. 7 illustrates a graph of V_(CT) (T_(PW) ⁺) versus C_(L), accordingto equation (3). Note that for any point on the graph, an increase inHelmholtz capacitance C_(L) results in a decrease in V_(CR) (T_(PW) ⁺).For example, point 700 represents C_(L) =10 μF, V_(CT) (T_(PW) ⁺) 4.06.It can be seen that for any CL>10 μF, V_(CT) (T_(PW) ⁺) <4.06. Forinstance, C_(L) 15 μF and V_(CT) (T_(PW) ⁺) =4.02 at point 705. Thus,V_(CT) (T_(PW) ⁺) of equation (3) is said to monotonically decrease inHelmholtz capacitance C_(L). It follows that any measured tank capacitorvoltage V_(CT) (T_(PW) ⁺) corresponds to a unique Helmholtz capacitanceC_(L) which may be calculated using the alternative embodimentspresented herein.

After the processor 470 calculates the lead/tissue resistance R_(L)using shunt resistor voltage measurement V_(RT) (0³⁰ ) in equation (1 ),all the variables in equation (3) are known except for the Helmholtzcapacitance C_(L). To determine C_(L), note that the right-hand side ofequation (3) consists of an additive term ##EQU4## a constant term##EQU5## and an exponential term ##EQU6## Because the Helmholtzcapacitance C_(L) is present in the additive, constant, and exponentialterms in equation (3), there is no explicit algebraic solution forC_(L). Hence, in one alternative embodiment, the processor 470 eithergenerates or retrieves from memory a set of candidate estimates forHelmholtz capacitance C_(L). The processor then evaluates theright-hand-side of equation (3) using each of the candidate estimates,recording the evaluation results into memory as a lookup table. Theprocessor 470 estimates C_(L) by determining which evaluation ofequation (3) most closely matches the voltage V_(CT) (T_(PW) ⁺) at theoutput of sample-and-hold unit U1. Because V_(CT) (T_(PW) ³⁰ ) inequation (3) decreases monotonically in C_(L), the value of C_(L) usedin equation (3) to compute the V_(CT) (T_(PW) ⁺) which most closelymatches the V_(CT) (T_(PW) ³⁰ ) measured from U1 is a good estimate ofthe actual Helmholtz capacitance, C_(L). Further, the processor 470 maybe programmed to estimate the Helmholtz capacitance to any arbitrarydegree of accuracy in this embodiment by evaluating equation (3) usingnumerous candidate values of C_(L) which are sufficiently closelyspaced.

Table I illustrates an exemplary lookup table using this alternativeembodiment. To generate Table I, processor 470 uses known values ofV_(I), C_(T), C_(B), R_(T), R_(SW), and T_(PW) which have beenpreviously stored in processor memory. For purposes of this example,these values are V_(I) =5 V, C_(T) =10 μF, C_(B) =10 μF, R_(T) =22.R_(SW) =17 Ω, and T_(PW) ⁺ =1.5 ms. Also, a set of candidate values forC_(L) has been stored into the processor 470. For purposes of thisexample, these values are 1 μF, 2 μF, 3 μF, 4 μF, 5 μF, 6 μF, 7 μF, 8μF, 9 μF, and 10 μF. Assuming also for this example that the processoruses the output of sample-and-hold unit U3 to calculate the lead/tissueresistance R_(L) =500 Ω, the processor evaluates equation (3) using eachof the candidate values of C_(L). Table I illustrates the resultingcalculations of V_(CT) (T_(PW) ³⁰ ) as a function of the candidate C_(L)values.

Table l: Example lookup table calculated from equation (3) and used toestimate C_(L)

    ______________________________________                                        C.sub.L       V.sub.CT (T.sub.PW.sup.+)                                         (candidate) (calculated)                                                    ______________________________________                                        1 μF       4.5981 V                                                          2 μF 4.3875 V                                                              3 μF 4.2750 V                                                              4 μF 4.2065 V                                                              5 μF 4.1606 V                                                              6 μF 4.1279 V                                                              7 μF 4.1033 V                                                              8 μF 4.0843 V                                                              9 μF 4.0690 V                                                              10 μF  4.0565 V                                                          ______________________________________                                    

In this example, the processor 470 measures from sample-and-hold unit U1the actual tank capacitor voltage after the pulse, or V_(CT) (T_(PW) ⁺),as 4.08 V. Scanning through the lookup table, the processor determinesthat the measured value of V_(CT) (T_(PW) ⁺) most closely matches thelookup table value 4.0843 V. Because C_(L) =8 μF corresponds to V_(CT)(T_(PW) ⁺)=4.0843, the processor determines C_(L) to be 8 μF in thisexample. Note that the impedance values, voltages, pulse width, andcandidate C_(L) values described herein are used only for this exampleand are not intended to limit the present invention. Furthermore, alookup table of this embodiment may have any number and range ofcandidate C_(L) values and should not be limited to the candidate C_(L)values presented in the example.

In another alternative embodiment, the processor 470 calculates thelead/tissue resistance R_(L) and measures the tank capacitor voltagefollowing the pacing pulse V_(CT) (T_(PW) ⁺) as before. In thisembodiment, however, the processor uses equation (3) to iterativelyestimate the Helmholtz capacitance C_(L). First, the processor 470substitutes an empirical estimate, preferably greater than the largestpossible Helmholtz capacitance C_(L), into the exponential term ofequation (3). The processor then solves for an approximation of C_(L) inthe additive and constant terms. If the empirical estimate of C_(L)agrees closely with the calculated approximation, then the processoruses the calculated approximation for the Helmholtz impedance.

The flowchart of FIG. 8 illustrates the steps of successiveapproximation involved in this embodiment if the processor inserts theempirical estimate of C_(L) into the exponential term of equation (3)and solves for an approximation of C_(L) using the additive and constantterms. The flowchart begins at the "start" block. Moving to block 800,the processor 470 computes the value of the exponential term ##EQU7##using an initial empirical estimate of C_(L), or C_(L) (empirical), thatis preferably larger than the largest possible C_(L) value. Using thecalculated E, equation (3) may be expressed as in equation (4), below,which permits solving for C_(L) algebraically. ##EQU8##

In block 805, the processor 470 solves equation (4) algebraically forC_(L), resulting in an approximation of the Helmholtz capacitance C_(L)(approx). The algebraic solution for C_(L) in equation (4) is given byC_(L) (approx) in equation (5): ##EQU9##

In block 810, the processor computes the absolute difference betweenC_(L) (empirical) and C_(L) (approx), or |C_(L) (empirical)-C_(L)(approx)|. If the absolute difference between C_(L) (empirical) andC_(L) (approx) is greater than a predetermined limit Δ_(CL), which ispreferably Δ_(CL) =1 μF, then the processor 470 moves to block 815 andadjusts the empirical estimate C_(L) (empirical) so that the absolutedifference between C_(L) (empirical) and C_(L) (approx) is smallerduring a subsequent iteration. Because of the nature of this procedure,equation (5) always produces a value of C_(L) (approx) that is betweenC_(L) (empirical) and the true Helmholtz capacitance. Thus, C_(L)(empirical) is preferably adjusted by setting C_(L) (empirical) equal toC_(L) (approx), although other known methods of adjusting C_(L)(empirical) so that C_(L) (empirical) and C_(L) (approx) convergeiteratively may be used as well. When C_(L) (empirical) is adjusted toproduce a new C_(L) (empirical) in step 815, the processor 470 repeatssteps 800, 805, 810, and 815 of the flowchart until C_(L) (approx) iswithin the predetermined limit AC_(L) of C_(L) (empirical).

Next moving to step 820, the processor 470 determines if C_(L)(empirical) is greater than C_(L) (approx). If C_(L) (empirical) isgreater than C_(L) (approx) in step 820, then the current C_(L)(empirical) is larger than the true Helmholtz capacitance, and theprocessor moves to step 825. In step 825, C_(L) (empirical) ispreferably adjusted by subtracting AC_(L) from C_(L) (empirical). Movingnext to step 830, the processor 470 computes the value of theexponential term E as in step 800, using the updated C_(L) (empirical).From the calculated E, equation (3) may be expressed as in equation (4),which permits solving for C_(L) algebraically. Hence, in block 835, theprocessor 470 solves equation (4) algebraically for C_(L) to obtain anupdated C_(L) (approx). As in step 805, the algebraic solution for C_(L)in step 835 is given by C_(L) (approx) in equation (5).

Next moving to step 840, the processor 470 determines if C_(L)(empirical) is less than or equal to C_(L) (approx). Because step 835always results in a C_(L) (approx) that is between C_(L) (empirical) andthe true Helmholt capacitance, the condition C_(L) (empirical) <C_(L)(approx) indicates that the previous adjustment of C_(L) (empirical) instep 825 resulted in a C_(L) (empirical) which was less than or equal tothe true Helmholtz capacitance. Accordingly, C_(L) (empirical) isguaranteed to be within Δ_(CL) below the true Helmholtz capacitance, andC_(L) (approx) is guaranteed to be between C_(L) (empirical) and thetrue Helmholtz capacitance. The processor thus moves to step 845, wherethe Helmholtz capacitance is estimated as C_(L) =C_(L) (approx).Alternatively, the Helmholtz capacitance may be estimated using theprevious value of C_(L) (approx), which is guaranteed to be withinΔ_(CL) above the true Helmholtz capacitance. If C_(L) (empirical) >C_(L)(approx) in step 840, however, then the processor repeats back to step825 to further adjust C_(L) (empirical).

Again examining step 820, if C_(L) (empirical)≦C_(L) (approx), thenC_(L) (empirical) is less than or equal to the true Helmholtzcapacitance, and the processor moves to step 850. From step 850, theprocessor 470 compares C_(L) (empirical) to C_(L) (approx). If C_(L)(empirical)=C_(L) (approx), then both C_(L) (empirical) and C_(L)(approx) are equal to the true Helmholtz capacitance, and the processor470 preferably estimates the Helmholtz capacitance as C_(L) (approx) instep 845. Alternatively, the processor 470 estimates the Helmholtzcapacitance as C_(L) (empirical) in step 845. In addition, the Helmholtzcapacitance may be estimated in step 845 as either the current orprevious value of C_(L) (empirical), since these values are guaranteedto be within Δ_(CL) of the true Helmholtz capacitance. If C_(L)(empirical) is not equal to C_(L) (approx) in step 850, then theprocessor 470 moves to step 855. Steps 855 through 870 correspondapproximately to steps 825 through 840, except that C_(L) (empirical) isassumed to be less than the true Helmholtz capacitance in steps 855through 870 and is therefore adjusted in step 855 by adding Δ_(CL) toC_(L) (empirical).

Following step 855, the processor 470 moves to step 860 to compute thevalue of the exponential term E as in step 800, using the updated C_(L)(empirical). From the calculated E, equation (3) may be expressed as inequation (4), which permits solving for C_(L) algebraically. Hence, inblock 865, the processor 470 solves equation (4) algebraically for C_(L)to obtain an updated C_(L) (approx). As in step 805, the algebraicsolution for C_(L) in step 865 is given by C_(L) (approx) in equation(5).

Next moving to step 870, the processor 470 determines if C_(L)(empirical) is greater than or equal to C_(L) (approx). Because step 865always results in a C_(L) (approx) that is between C_(L) (empirical) andthe true Helmholtz capacitance, the condition C_(L) (empirical) ≧C_(L)(approx) indicates that the previous adjustment of C_(L) (empirical) instep 855 resulted in a C_(L) (empirical) which was greater than or equalto the true Helmholtz capacitance. Accordingly, C_(L) (empirical) isguaranteed to be within Δ_(CL) above the true Helmholtz capacitance, andC_(L) (approx) is guaranteed to be between C_(L) (empirical) and thetrue Helmholtz capacitance. The processor thus moves to step 845, wherethe Helmholtz capacitance is estimated as C_(L) =C_(L) (approx).Alternatively, the Helmholtz capacitance may be estimated using theprevious value of C_(L) (approx), which is guaranteed to be C_(L) belowthe true Helmholtz capacitance. In addition, the Helmholtz capacitancemay be estimated in step 845 as either the current or previous value ofC_(L) (empirical), since these values are guaranteed to be within Δ_(CL)of the true Helmholtz capacitance. If C_(L) (empirical) <C_(L) (approx)in step 870, however, then the processor repeats back to step 855 tofurther adjust C_(L) (empirical).

When the Helmholtz capacitance C_(L) and load resistance R_(L) have beendetermined, a plurality of parameters of importance for analyzing andoptimizing a pacing system may be calculated, including the currentdelivered to the cardiac tissue at any instantaneous point in time, theaverage current delivered to the cardiac tissue over the duration of thepulse, the total charge and the total energy delivered to the cardiactissue and to the leads, and the Helmholtz potential after pacingpolarization. For instance, the current flowing through the heart tissueat time t, or i_(L) (t), is given by equation (6), neglecting R_(X) :##EQU10## where e is the base of the natural logarithm.

Neglecting R_(X) as before, equation (7) represents the average currentflowing through the heart tissue: ##EQU11## where e is the base of thenatural logarithm.

Again neglecting R_(X), equation (8) represents the charge Q_(D)delivered to the heart tissue from time t=0 to time t=T_(PW) : ##EQU12##where e is the base of the natural logarithm.

Finally, the energy J_(D) delivered to the heart tissue from time t=0 totime t=T_(PW), neglecting R_(X) as before, is given by equation (9).##EQU13##

Thus, the present invention produces a very accurate impedancecharacterization of the lead/tissue interface, including both resistiveand reactive impedance components. Further, since buffers U4 and U5 havehigh-impedance inputs coupled directly to the pulse generator 468, thepresent invention is adapted to perform impedance measurements duringnormal pacing and defibrillating operation and with minimal interferenceto the pulse generator 468. In addition, and importantly, because theimpedance measurements occur during normal pacer operation, the paceroperation need not be suspended in order to collect impedance data.

Because the processor 470 controls the switches SW1, SW2, and SW3 andalso the sample signals, the processor 470 may be easily programmed tocalculate lead/tissue impedance whenever desired. For instance, theprocessor 470 may calculate the lead/tissue impedance during everyn^(th) pacing pulse, where n can be an arbitrary integer. The periodicimpedance calculations can then be stored into main memory to beretrieved at a later date, perhaps by a physician who needs to verify oroptimize the implantable device 400. Storing the calculations in memoryalso allows the processor 470 to perform statistical analyses which areuseful for pacer maintenance, such as calculating minimum impedancemeasurements, maximum impedance measurements, and moving averages. Inaddition, if the implantable device 400 is capable of external controlthrough telemetry with a device external to the body, the processor 470can easily be programmed to calculate lead impedance duringmanually-induced test sequences. Hence, physicians have access to bothlong-term and immediate impedance data with which to optimize andmaintain the implanted device.

The alternative embodiments described above allow the processor 470 toaccurately calculate both the lead/tissue resistance R_(L) as well asthe Helmholtz capacitance C_(L) to any arbitrary degree of accuracy.Further, the alternative embodiments do not require measurement of theshunt resistor voltage V_(CT) (T_(PW) ⁻⁻) just prior to the end of thepulse at time t=T_(PW) ⁻⁻.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A method, comprising:determining a lead/tissueresistance; and using lead/tissue resistance to determine a Helmholtzcapacitance of a patient's heart.
 2. The method of claim 1 whereindetermining a lead/tissue resistance comprises:applying a voltage V_(i)to a closed-loop current path comprising a lead, heart tissue, andcomponents of known impedance; measuring a voltage produced in theclosed-loop current path by the voltage V₁ after applying the voltage V₁to closed-loop current path, thereby producing a measured voltage; andcalculating the lead/tissue resistance based on the measured voltage. 3.The method of claim 2 wherein measuring a voltage comprises measuring avoltage across a resistor, thereby producing the measured voltage acrossthe resistor.
 4. The method of claim 3 in which the lead/tissueresistance is calculated using a formula which comprises the measuredvoltage across the resistor as a variable of the formula.
 5. The methodof claim 1 wherein using the lead/tissue resistance to determine aHelmholtz capacitance of a patient's heart further comprises:applying avoltage V_(i) for a fixed amount of time, T_(PW) seconds, to aclosed-loop current path comprising a lead, heart tissue, and componentsof known impedance; measuring a voltage produced in the closed-loopcurrent path by the voltage V; at a time t=T_(PW) the time t=T_(PW)occurring just before time t=T_(PW), thereby producing a measuredvoltage; and calculating the Helmholtz capacitance based on the measuredvoltage.
 6. The method of claim 5 wherein the Helmholtz capacitance iscalculated using a formula which comprises the measured voltage as avariable of the formula.
 7. The method of claim 2 wherein using thelead/tissue resistance to determine a Helmholtz capacitance of apatient's heart further comprises:discharging a tank capacitor for aduration of T_(PW) seconds through a closed-loop current path comprisinga lead, heart tissue, and components of known impedance; measuring avoltage across the tank capacitor at a time t=T_(PW) ⁺ just after theend of the duration of T_(PW) seconds, thereby producing a measuredvoltage; and estimating the Helmholtz capacitance based on the measuredvoltage.
 8. The method of claim 7 wherein estimating the Helmholtzcapacitance includes calculating an empirical voltage value across thetank capacitor based on a predetermined empirical estimate of aHelmholtz capacitance C_(L) (empirical).
 9. The method of claim 8wherein estimating the Helmholtz capacitance further includescalculating additional empirical voltage values across the tankcapacitor based on additional predetermined empirical estimates of theHelmholtz capacitance C_(L) (empirical).
 10. The method of claim 9wherein estimating the Helmholtz capacitance further includes estimatingthe Helmholtz capacitance to be the empirical estimate C_(L) (empirical)which is associated with the empirical voltage value that is closest tothe measured voltage.
 11. The method of claim 7 wherein estimating theHelmholtz capacitance comprises an iterative process of calculating anempirical voltage value across the tank capacitor based on apredetermined empirical estimate of a Helmholtz capacitance C_(L)empirical), calculating an approximation of a Helmholtz capacitanceC_(L) (approx) based on the empirical voltage value, comparing C_(L)approx) with C_(L) (empirical) and adjusting C_(L) (empirical) for asubsequent iteration of the process, wherein the process is repeateduntil comparing C_(L) (approx) with C_(L) (empirical) indicates thatC_(L) (approx) is within a predetermined limit of C_(L) (empirical). 12.The method of claim 11 wherein adjusting C_(L) (empirical) comprisesadjusting C_(L) (empirical) by an amount equal to the predeterminedlimit.